VARIABLE GEOMETRY TURBINE

Description

[0001] The present invention relates to a variable geometry turbine, particularly, but not exclusively, for use in a turbocharger of an internal combustion engine.

[0002] Turbochargers are known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel that is mounted on the other end of the shaft and within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.

[0003] In known turbochargers, the turbine comprises a turbine chamber within which the turbine wheel is mounted, an inlet passageway defined between facing radial walls arranged around the turbine chamber, an inlet volute arranged around the inlet passageway, and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate in such a way that pressurised exhaust gas admitted to the inlet volute flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to trim turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.

[0004] Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level that ensures efficient turbine operation by reducing the size of the inlet passageway.

[0005] In one known type of variable geometry turbine, an axially moveable wall member, generally referred to as a "nozzle ring", defines one wall of the inlet passageway. The position of the nozzle ring relative to a facing wall of the inlet passageway is adjustable to control the axial width of the inlet passageway. Thus, for example, as gas flowing through the turbine decreases, the inlet passageway width may also be decreased to maintain gas velocity and to optimise turbine output. Such nozzle rings comprise a generally annular wall and inner and outer axially extending flanges. The flanges extend into a cavity defined in the turbine housing, which is a part of the housing that in practice is provided by the bearing housing, which accommodates axial movement of the nozzle ring.

[0006] The nozzle ring may be provided with vanes that extend into the inlet passageway and through slots provided on the facing wall of the inlet passageway to accommodate movement of the nozzle ring. Alternatively, vanes may extend from the fixed wall through slots provided in the nozzle ring. Generally the nozzle ring is supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator that axially displaces the rods. Various forms of actuators are known for use in variable geometry turbines, including pneumatic, hydraulic and electric actuators that are mounted externally of the turbocharger and connected to the variable geometry system via appropriate linkages.

[0007] When a conventional turbine is in use, with gas passing through the inlet passageway, pressure is applied to the face of the nozzle ring tending to force the nozzle ring into the annular cavity. The actuating mechanism must overcome the effect of any pressure difference across the nozzle ring if the position of the nozzle ring is to be controlled accurately. Moving the nozzle ring closer to the facing wall of the passageway, so as to further reduce the width of the passageway and increase the speed of the air flow, tends to increase the load applied to the face of the nozzle ring by the exhaust gases. Some actuators for turbines, for example electric actuators, are able to provide only a relatively limited force to move a nozzle ring when compared to pneumatic actuators. In some operating conditions, the force needed to be supplied by the actuator can exceed the capability of the actuator. Furthermore, it is also desirable to ensure that the resultant force on the nozzle ring is unidirectional.

[0008] It is known to provide balance apertures in the nozzle ring to reduce pressure differences across the nozzle ring and thereby to reduce the load applied to the face of the nozzle ring. For example, EP0654587 discloses a variable geometry turbine with pressure balance apertures in the nozzle ring between nozzle vanes. The forces on the nozzle ring are created by the pressure on the nozzle ring face, the pressure in the cavity behind the nozzle ring, and by the actuator. The function of the pressure balance apertures is to ensure that the cavity behind the nozzle ring is at a pressure substantially equal to, but always slightly less than, the pressure acting on the front face of the nozzle ring to ensure a small but unidirectional force on the nozzle ring. The turbine nozzle ring is provided with an annular array of vanes extending across the turbine inlet such that air flowing through the inlet flows radially between adjacent vanes that can be regarded as defining a vane passage. The turbine inlet has a reduced radial flow area in the region of the vane passage with the effect that the inlet gas speed increases through the vane passage with a corresponding drop in pressure in this region of the nozzle ring. Accordingly, the pressure balance holes as described in EP0654587 are located between vanes in the sense that the inner and/or outer extremity of each balance aperture lies within the inner or outer radial extent of the nozzle guide vane passage.

[0009] US2005/262841, GB2461720 and JP S60175707 all disclose variable geometry turbines. US2005/262841 discloses a variable geometry turbine having an annular inlet passageway defined between a radial wall of a moveable wall member and a facing wall of the turbine housing. The moveable wall member is mounted within an annular cavity provided within the housing and having inner and outer annular surfaces. An annular seal is disposed between an an annular flange of the moveable wall member and the adjacent inner or outer annular surface of the cavity. US2005/262841 does not disclose the particular geometry of the radial wall as in the present invention in order to solve the technical problems discussed below.

[0010] It may be desirable to provide a variable geometry turbine at least partially addresses one or more problems associated with known variable geometry turbines, whether identified herein or otherwise.

[0011] According to a first aspect of the present invention there is provided a variable geometry turbine as set forth in claim 1.

[0012] The axially extending apertures may be referred to as balance apertures in the moveable wall member and, in use, they serve to reduce pressure differences across the generally annular wall of the movable wall member and thereby reduce loads applied to the face of the generally annular wall of the movable wall member. The moveable wall member may be moveable between a fully opened position and a fully closed position. When disposed in the fully opened position, a portion of the base surface of the cavity may contact a portion of the movable wall member.

[0013] Since the profile shape of the base surface generally matches the profile shape of the interior surface of the movable wall member, the volume within the cavity which can be filled with gas is significantly reduced with respect to known arrangements. For example, in known arrangements the cavity is typically formed as a generally annular channel extending axially into an axially facing surface of the housing, comprising: a radially inner curved wall, a radially outer curved wall and a generally flat base wall. Similarly, in known arrangements the interior surface of the movable wall member is typically defined by a generally annular channel defined by an inner surface of the radially inner flange, an inner surface of the radially outer flange and a generally flat inner surface of the generally annular wall. In addition, the interior surface of the movable wall member is typically further defined by two supports. These supports, in the form of push rods, are typically attached to the inner surface of the generally annular wall and typically extend through apertures in the generally flat base wall of the cavity for connection to an actuation mechanism. With such a prior art arrangement, the base surface of the cavity is generally flat and the interior surface of the movable wall member is partly defined by a generally flat inner surface of the generally annular wall, in combination with the two supports. Therefore, with such a prior art arrangement, when the moveable wall member is disposed in the fully opened position, apart from the space taken up by the two support, the entire volume of the generally annular channel defined by inner surfaces of the radially inner flange, the radially outer flange and the generally annular wall can be filled with gas.

[0014] Therefore, the variable geometry turbine according to the first aspect of the invention provides of an arrangement with balance apertures that can reduce pressure differences across the movable wall member (i.e. pressure differences between the gas flow through the inlet and the cavity in the housing) whilst reducing the available volume that can support gas within the cavity. This is particularly advantageous for situations which, in use, will encounter large fluctuations of pressure within the inlet, as now discussed.

[0015] It is known that for a turbocharger that is, in use, connected to an engine the exhaust gas that flows through the turbine (which may be, for example, a variable geometry turbine) will comprise a plurality of pulses, each pulse originating from a different cylinder of the engine. As a result, the pressure within the turbine inlet fluctuates due to the timing of the exhaust pulses received from the exhaust manifold of the vehicle engine. This pressure fluctuation is present both when the turbocharger is operating in an engine "fired" mode and also an engine "braking" mode. For instance, in braking mode the pressure fluctuation can give rise to an undesirable fluctuation in the braking torque produced. The terms "fired" mode and "braking" mode are well known to the ordinarily skilled artisan in this field.

[0016] The inventors of the present invention have realised that for such a time varying pressure in the turbine inlet, although the balance apertures in the movable wall member allow the pressure in the cavity behind the movable wall member to equalise the local pressure in the inlet proximate to the balance apertures, there is a time lag between the pressure in the cavity and the local pressure in the inlet proximate to the balance apertures. It will be appreciated that the average pressure in the cavity behind the movable wall member will be substantially equal to the local average pressure in the inlet proximate to the balance apertures. However, as the instantaneous pressure in the inlet proximate to the balance apertures varies with time (due to the timing of the exhaust pulses), the instantaneous pressure in the cavity behind the movable wall member also vary with time in a similar way but with a lag (or phase difference) with respect to the instantaneous pressure in the inlet proximate to the balance apertures. For sufficiently high frequency pressure variations this time lag can result in large time varying loads being applied to the movable wall member (which loads must be overcome by the actuating mechanism in order to accurately control position of the movable wall member). Furthermore, the inventors of the present invention have realised that this time lag (which represents the time taken to fill or evacuate the cavity to equalise pressure across the balance apertures) is dependent on the volume of the cavity which is filled with gas.

[0017] Since the profile shape of the base surface generally matches the profile shape of the interior surface of the movable wall member, the volume within the cavity of the variable geometry turbine according to the first aspect of the invention which can be filled with gas is significantly reduced with respect to known arrangements. In turn, advantageously, this reduces the magnitude of the peak to peak variation in the loads that are applied to the movable wall member and which must be overcome by the actuating mechanism in order to accurately control position of the movable wall member.

[0018] Furthermore, the variable geometry turbine according to the first aspect of the invention reduces the magnitude of the time varying loads that are applied to the movable wall member and which must be overcome by the actuating mechanism without adversely affecting the efficiency of the turbine. In fact, the variable geometry turbine according to the first aspect of the invention can reduce the magnitude of these time varying loads and, in addition, may increase the efficiency of the turbine over known arrangements, as now discussed.

[0019] The turbine nozzle ring is usually provided with an array of vanes extending across the turbine inlet. Air flowing through the inlet flows radially between adjacent vanes that can therefore be regarded as defining a vane passage. The turbine inlet has a reduced radial flow area in the region of the vane passage with the effect that the inlet gas speed increases through the vane passage with a corresponding drop in pressure in this region of the nozzle ring. Accordingly, the pressure balance holes as described in EP0654587 are located between vanes in the sense that the inner and/or outer radial extremity of each balance aperture lies within the inner or outer radial extent of the nozzle guide vane passage.

[0020] It has been previously found that even with the provision of pressure balance holes as disclosed in EP0654587, the force on the nozzle ring can fluctuate undesirably as the pressure within the turbine inlet fluctuates due to exhaust pulses being released into the exhaust manifold of the vehicle engine. In order to reduce the magnitude of load variations on the movable wall member produced by these pressure fluctuations, in EP1888881 it has been proposed to provide, in combination with the balance apertures taught in EP0654587 (herein referred to as primary balance apertures), additional balance apertures (herein referred to as peripheral balance apertures) either upstream or downstream of the primary balance apertures. In particular, the provision of peripheral balance apertures upstream of (i.e. at a larger radius than) the primary balance apertures can result in a reduction in the force amplitude at the actuator interface caused by an exhaust pulse passing through the turbine stage when compared with the provision of primary pressure balance apertures, alone.

[0021] However, the variable geometry turbine according to the first aspect of the invention has a number of advantages over the arrangement disclosed in EP1888881, as now discussed.

[0022] It will be appreciated that as gas flows through the inlet passageway the pressure of the gas flow drops as it moves across the face of the nozzle ring towards the turbine wheel. Therefore, by selecting a particular radial position for the balance apertures, an average pressure within the cavity (which will be substantially equal to the local average pressure in the inlet proximate to the balance apertures) can be maintained. The provision of peripheral balance apertures upstream of (i.e. at a larger radius than) the primary balance apertures will have the effect of increasing the average pressure within the cavity behind the movable wall member. In turn, this reduces the range of average pressures that can be achieved by selection of a radial position of the primary balance apertures. Put differently, once the peripheral balance apertures have been added, in order to achieve the same average pressure within the cavity behind the movable wall member as was achieved without them, the primary balance apertures have to be moved to a lower pressure region (i.e. to a small radius with respect to the turbine axis).

[0023] Furthermore, in contrast the variable geometry turbine according to the first aspect of the invention may not need such secondary balance apertures or, alternatively, may be provided with fewer such secondary balance apertures. It will be appreciated that such secondary balance apertures represent a leak path within the turbine. Therefore, since the variable geometry turbine according to the first aspect of the invention does not need such secondary balance apertures the efficiency of the turbine will be increased relative to this prior art arrangement (as taught by EP1888881). In fact, since the profile shape of the base surface generally matches the profile shape of the interior surface of the movable wall member, the volume within the cavity which can be filled with gas is significantly reduced with respect to known arrangements. With such a reduced volume within the cavity which can be filled with gas, a smaller total area of the balance apertures can be used with respect to known turbines to achieve the same level of balancing. In turn, this can result in an increase in the efficiency of the turbine with respect to the arrangement of EP0654587 and a further increase in efficiency with respect to the arrangement of EP1888881.

[0024] The interior surface of the movable wall member may be at least partially defined by inner surfaces of the generally annular wall and the radially inner and outer flanges.

[0025] The movable wall member may further comprise at least one support. For example, the movable wall member may comprise two supports, each support being of the form of a push rod. The interior surface of the movable wall member may be at least partially defined by said at least one support and any connecting members or connecting portions of said at least one support. For example, each support may be connected to a main body of the movable wall member (which may be referred to as a nozzle ring) via an arcuate connecting member. Said connecting members and supports at least partially define the interior surface of the movable wall member.

[0026] At least part of the base surface of the cavity and at least part of the interior surface of the movable wall member may be not flat.

[0027] One of the base surface of the cavity and the interior surface of the movable wall member may be at least partially generally concave and the other may be at least partially generally convex. It may be that the generally convex shape can be partially received within the generally concave shape.

[0028] Each arcuate radially central portion may be of the form of an axial protrusion from a generally flat portion of the base surface.

[0029] The number of arcuate radially central portions may be dependent on the number of supports (for example push rods) that the movable wall member has. Each arcuate radially central portion may extend circumferentially generally between apertures that supports of the movable wall member extend through.

[0030] Along its circumferential extent, each arcuate radially central portion may comprise two end portions and a central portion disposed there between. The axial extent of the central portion may be greater than that of the two end portions. Adjacent end portions of two arcuate radially central portions may be separated by an aperture through which a support of the movable wall member extends and the reduced axial extent of two end portions relative to the central portion may form a void that accommodates a connecting member or portion of said support.

[0031] The movable wall member may support an array of circumferentially spaced inlet vanes each of which extends across the inlet passageway. At least some of the axially extending apertures provided through the generally annular wall of the moveable member may be located between the inlet vanes.

[0032] The moveable wall member may be moveable between a fully open position and a fully closed position. When disposed in the fully open position, part of the moveable member may contact part of the base surface of the cavity. For example, when disposed in the fully open position, the radially inner and outer flanges of the moveable member may contact a portion of the base surface of the cavity.

[0033] The base surface of the cavity and the interior surface of the movable wall member may be formed from materials that are impermeable to gas flow.

[0034] The shape of the base surface of the cavity and the profile shape of the interior surface of the movable wall member may be such that the volume of the cavity is reduced by at least 20% relative to an arrangement wherein the base surface of the cavity and the interior surface of the generally annular wall were both flat.

[0035] According to a second aspect of the present invention there is provided a turbocharger comprising the variable geometry turbine according to the first aspect of the invention. According to a third aspect of the present invention there is provided a method of forming a variable geometry turbine as set forth in claim 14.

[0036] The variable geometry turbine formed according to the third aspect of the invention may have any of the features of the variable geometry turbine according to the first aspect of the invention as desired.

[0037] Providing the housing having the cavity may comprise casting a part of the housing on which the cavity is formed. The part of the housing on which the cavity is formed may be a bearing housing.

[0038] Providing the housing having the cavity may further comprise machining the casting to form at least a part of the cavity.

[0039] Additionally or alternatively, providing the housing having the cavity may further comprise attaching to the casting one or more additional members, the one or more additional members contributing to the profile shape of the base surface of the cavity.

[0040] Specific embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, of which:

Figure 1 is a cross-section of a turbocharger incorporating a variable geometry turbine in accordance with an embodiment of the present invention;

Figure 2a is an enlarged portion of the cross-section shown in Figure 1, showing details of the movable wall member and cavity according to an embodiment of the present invention;

Figure 2b is a cross-section similar to that shown in Figure 2a but showing details of a known movable wall member and cavity;

Figure 3a is a first perspective view of the movable wall member shown in Figures 1 and 2a;

Figure 3b is a second perspective view of the movable wall member shown in Figures 1 and 2a;

Figure 4a is a perspective view of an axial end of the bearing housing of the turbocharger shown in Figures 1 and 2a, which defines a cavity for receipt of radially inner and outer flanges of the moveable member;

Figure 4b is a perspective view of an axial end of the bearing housing of the known turbocharger shown in Figure 2b, which defines a cavity for receipt of radially inner and outer flanges of a moveable member;

Figure 5 shows a plot of the volume in the cavity behind the movable wall member as a function of the axial gap between the generally annular wall and the shroud for both: (a) the embodiment shown Figures 1, 2a, 3a, 3b and 4a;

and (b) the known arrangement shown in Figures 2b and 4b, and a plot of the volume reduction (as a percentage) of the cavity relative the known cavity as a function of the axial gap between the generally annular wall and the shroud;

Figure 6a a cross-section showing details of the movable wall member and cavity according to an embodiment of the present invention, with the movable wall member disposed in a closed position;

Figure 6b a cross-section showing details of the movable wall member and cavity according to an embodiment of the present invention, with the movable wall member disposed between a closed position and an open position;

Figure 6c a cross-section showing details of the movable wall member and cavity according to an embodiment of the present invention, with the movable wall member disposed in an open position;

Figure 7 shows plots of both the load on the movable wall member and the efficiency of the variable turbine as a function of time under specific engine conditions for both: (a) the embodiment shown Figures 1, 2a, 3a, 3b and 4a (dashed lines); and (b) the known arrangement shown in Figures 2b and 4b (solid lines); and

Figure 8 shows the reduction factor of the amplitude of the time varying component of the load on the movable wall member plotted against the volume reduction (relative to the known arrangement shown in Figures 2b, 4b) for 9 different points in the space of operating conditions and geometry of the base wall.

[0041] An embodiment of a turbocharger 1 incorporating a variable geometry turbine in accordance with an embodiment of the present invention is now described with reference to Figures 1, 2a, 3a, 3b and 4a.

[0042] Figure 1 shows a turbocharger 1 incorporating a variable geometry turbine in accordance with an embodiment of the present invention. The turbocharger 1 comprises a turbine housing 2 and a compressor housing 3 interconnected by a central bearing housing 4. A turbocharger shaft 5 extends from the turbine housing 2 to the compressor housing 3 through the bearing housing 4. A turbine wheel 6 is mounted on one end of the shaft 5 for rotation within the turbine housing 2, and a compressor wheel 7 is mounted on the other end of the shaft 5 for rotation within the compressor housing 3. The shaft 5 rotates about turbocharger axis 8 on bearing assemblies located in the bearing housing 4.

[0043] It will be appreciated that the turbine housing 2 and an axial end of the bearing housing 4 together form a housing of the variable geometry turbine, in which the turbine wheel 6 is supported for rotation about turbocharger axis 8.

[0044] The turbine housing 2 defines an inlet volute 9 to which exhaust gas from an internal combustion engine (not shown) is delivered. The exhaust gas flows from the inlet volute 9 to an axial outlet passage 10 via an inlet passageway 11 and the turbine wheel 6. The inlet passageway 11 is defined between two axially spaced walls. In particular, the inlet passageway 11 is defined on one side by a face of a movable wall member 12, commonly referred to as a "nozzle ring," and on the opposite side by a shroud 13. The shroud 13 covers the opening of a generally annular recess 14 in the turbine housing 2.

[0045] As will be appreciated by the skilled person, the inlet volute 9 may comprise a generally toroidal volume (defined by the turbine housing 2) and an inlet arranged to direct exhaust gas from an internal combustion engine tangentially into the generally toroidal volume. As exhaust gas enters the inlet volute 9 it flows circumferentially around the generally toroidal volume and radially inwards towards the inlet passageway 11. In the vicinity of the inlet, there is provided a wall or "tongue" 18 which serves to separate the generally toroidal volume in the vicinity of the inlet of the volute 9 from the inlet passageway 11 of the turbine. The tongue 18 may help to guide the exhaust gas circumferentially around the generally toroidal volume and may also aid the mixing of the generally linear gas flowing into the volute 9 with the circumferential gas flow around the generally toroidal volume. In the cross section shown in Figure 1, the tongue 18 is visible on one side of the axis 8 only.

[0046] Figures 3a and 3b show two different perspective views of the movable wall member 12.

[0047] The movable wall member 12 supports an array of circumferentially and equally spaced inlet vanes 15 each of which extends across the inlet passageway 11. The vanes 15 are orientated to deflect gas flowing through the inlet passageway 11 towards the direction of rotation of the turbine wheel 6. The shroud 13 is provided with suitably configured slots for receipt of the vanes 15 such that as the movable wall member 12 moves axially towards the shroud 13, a distal end of each of the vanes 15 moves through one of said slots and protrudes into the recess 14.

[0048] Accordingly, by appropriate control of the actuator (which may for instance be pneumatic or electric), the axial position of the movable wall member 12 can be controlled. The speed of the turbine wheel 6 is dependent upon the velocity of the gas passing through the inlet passageway 11. For a fixed rate of mass of gas flowing into the inlet passageway 11, the gas velocity is a function of the width of the inlet passageway 11, the width being adjustable by controlling the axial position of the movable wall member 12. As the width of the inlet passageway 11 is reduced, the velocity of the gas passing through it increases. Figure 1 shows the nozzle ring 12 disposed between a fully open position and a fully closed position such that the width of inlet passageway 11 is greater that a minimum width and smaller than a maximum width.

[0049] Gas flowing from the inlet volute 9 to the outlet passage 10 passes over the turbine wheel 6 and as a result torque is applied to the shaft 5 to drive the compressor wheel 7. Rotation of the compressor wheel 7 within the compressor housing 3 pressurises ambient air present in an air inlet 16 and delivers the pressurised air to an air outlet volute 17 from which it is fed to an internal combustion engine (not shown).

[0050] The movable wall member (or nozzle ring) 12 comprises a generally annular wall 20 and radially inner and outer flanges 21, 22 extending axially from the generally annular wall 20.

[0051] A cavity 25 is provided in the housing of the variable geometry turbine for receipt of the radially inner and outer flanges 21, 22 of the moveable member 12. It will be appreciated that the cavity 25 is formed on an axial end of the bearing housing 4, which cooperates with the turbine housing 2 to form the housing of the variable geometry turbine. Figure 4a shows a perspective view of the axial end of the bearing housing 4 of the turbocharger 1, which defines the cavity 25.

[0052] As the movable wall member 12 moves axially, the extent to which the radially inner and outer flanges 21, 22 of the moveable member 12 are received in the cavity 25 varies. The cavity 25 is defined by radially inner and outer curved side surfaces 26, 27 and a base surface 28 extending between the radially inner and outer curved side surfaces 26, 27. The moveable wall member 12 is moveable between a fully opened position and a fully closed position. When disposed in the fully opened position, the radially inner and outer flanges 21, 22 of the moveable member 12 may contact a portion of the base surface 28 of the cavity 25. That is, a portion of the base surface 28 of the cavity 25 may serve as a physical stop to limit the range of axial movement of the moveable member 12.

[0053] Inner and outer sealing rings 30 and 31 are provided to seal the movable wall member 12 with respect to inner and outer curved surfaces 26, 27 of the cavity 25 respectively, whilst allowing the movable wall member 12 to slide within the cavity 25. The inner sealing ring 30 is supported within an annular groove formed in the radially inner curved surface 30 of the cavity 25 and bears against the inner flange 21 of the movable wall member 12. The outer sealing ring 31 is supported within an annular groove formed in the radially outer curved surface 27 of the cavity 25 and bears against the outer flange 22 of the movable wall member 12.

[0054] As can be seen in Figures 3a and 3b, a plurality of axially extending apertures 32, 33 provided through the generally annular wall 20 of the moveable wall member 12. The apertures 32, 33 may be referred to as balancing apertures 32, 33. The apertures 32, 33 connect the inlet 11 to the cavity 25, such that the inlet 11 and the cavity 25 are in fluid communication via the apertures 32, 33. In use, the apertures 32, 33 serve to reduce pressure differences across the generally annular wall 20 of the movable wall member 12 and thereby reduce loads applied to the face of the generally annular wall 20 of the movable wall member 12.

[0055] It will be appreciated that as gas flows through the inlet passageway 11 the pressure of the gas flow drops as it moves across the face of the movable wall member 12 towards the turbine wheel 6. Therefore, by selecting a particular radial position for the balance apertures 32, 33, an average pressure within the cavity 25 (which will be substantially equal to the local average pressure in the inlet 11 proximate to the balance apertures 32, 33) can be maintained.

[0056] In use, as air flows radially inwards through the turbine inlet 11, it flows between adjacent vanes 15, which can be regarded as defining a vane passage. The turbine inlet 11 has a reduced radial flow area in the region of the vane passage with the effect that the inlet gas speed increases through the vane passage with a corresponding drop in pressure in this region of the movable wall member 12. Accordingly, a first set of balancing aperture 32 are located between pairs of adjacent vanes 15 in the sense that the inner and outer radial extremity of these balancing apertures 32 lie within the inner or outer radial extent of the vane passage. In this embodiment, a balancing aperture 32 is located between each pair of adjacent vanes 15.

[0057] In addition, in this embodiment, a smaller number of balancing apertures 33 are provided upstream of (i.e. at a larger radius than) the balance apertures 32 located between pairs of adjacent vanes 15. These balance apertures 33 can result in a reduction in the force amplitude at the actuator interface caused by an exhaust pulse passing through the inlet passageway 11 when compared with the provision of the balance apertures 32 located between pairs of adjacent vanes 15 alone. As discussed further below, the profile shape of the base surface 28 of the cavity 25 generally matches an interior surface of the movable wall member 12, which also reduces the magnitude of the time varying loads that are applied to the movable wall member 12. Therefore, it will be understood that although the described embodiment comprises balance apertures 33 that are upstream of the balance apertures 32 located between pairs of adjacent vanes 15, these balance apertures 33 are optional. In other, alternative embodiments, these apertures 33 may be absent.

[0058] The movable wall member 12 further comprises two supports 34, each of the supports being generally of the form of a shaft or rod. The two supports 34 may be referred to as push rods. Each of the two supports 34 is attached to the inner surface of the generally annular wall 20 (i.e. the surface that is distal from the inlet 11) via an arcuate connecting member 35. The connection between each of the two supports 34 and the inner surface of the generally annular wall 20 may, for example, be generally of the form described in EP0917618.

[0059] The supports 34 extend through apertures 36 in the base surface 28 of the cavity 25 for connection to an actuation mechanism. The position of the movable wall member 12 is controlled by an actuator assembly, which may be generally of the type disclosed in US 5,868,552. An actuator (not shown) is operable to adjust the position of the movable wall member 12 via a mechanical linkage. For example, an actuator may be connected by a lever system to a bar upon which a generally C-shaped yoke is mounted. The ends of the generally C-shaped yoke may engage with the two supports 34 via notches 37.

[0060] Inner surfaces of the generally annular wall 20 and radially inner and outer flanges 21, 22 define an interior surface 38 of the movable wall member 12.

[0061] The interior surface 38 of the movable wall member 12 is defined by a generally annular channel defined by an outer surface of the radially inner flange 21, an inner surface of the radially outer flange 22 and a generally flat inner surface of the generally annular wall 20. In addition, the interior surface 38 of the movable wall member is further defined by the two supports 34 and the two arcuate connecting members 35.

[0062] As can be best seen from Figures 3b and 4a, a profile shape of the base surface 28 of the cavity 25 in the housing of the variable geometry turbine generally matches a profile shape of the interior surface 38 of the movable wall member 12.

[0063] In order to achieve this, the base surface 28 extending between the radially inner and outer curved side surfaces 26, 27 is not flat. Rather, the base surface comprises two arcuate radially central portions 40 which are shaped so as to be received in the interior of the moveable wall member 12 when it is disposed in the fully opened position. Each arcuate radially central portion 40 is of the form of an axial protrusion from a generally flat portion 39 of the base surface 28 at an axial end surface of the bearing housing 4. Each arcuate radially central portion 40 is defined by radially inner and outer curved surfaces 41, 42. Each arcuate radially central portion 40 extends circumferentially generally between the two apertures 36 that the supports 34 extend through.

[0064] When disposed in the fully opened position, the radially inner flange 21 of the moveable member 12 is received in a groove formed between the radially inner curved side surface 26 of the cavity 25 and radially inner curved surface 41 of the arcuate radially central portion 40. Similarly, when disposed in the fully opened position, the radially outer flange 22 of the moveable member 12 is received in a groove formed between the radially outer curved side surface 27 of the cavity 25 and radially outer curved surface 42 of the arcuate radially central portion 40. When disposed in the fully opened position, the radially inner and outer flanges 21, 22 of the moveable wall member 12 contact the flat portion 39 of the base surface 28 of the cavity 25. That is, this flat portion 39 of the base surface 28 of the cavity 25 serves as a physical stop to limit the range of axial movement of the moveable member 12. Although in this embodiment, the flat portion 39 of the base surface 28 of the cavity 25 serves as a physical stop to limit the range of axial movement of the moveable member 12, it will be appreciated that in alternative embodiments, any other part of the base surface 28 of the cavity 25 may serve as a physical stop to limit the range of axial movement of the moveable member 12. For example, in some embodiments, when the movable wall member 12 is disposed in the fully opened position, the generally annular wall 20 may contact the arcuate radially central portions 40. In general, when disposed in the fully opened position, part of the moveable wall member 12 may contact part of the base surface 28 of the cavity 25.

[0065] Along its circumferential extent, each arcuate radially central portion 40 comprises two end portions 43 and a central portion 44 disposed therebetween. The axial extent of the central portion 44 is greater than that of the two end portions 43. The adjacent end portions 43 of the two arcuate radially central portions 40 are separated by one of the apertures 36 that the supports 34 extend through. It will be appreciated that the reduced axial extent of two end portions 43 (relative to the central portion 44) forms a void that accommodates the arcuate connecting members 35 that facilitate the connection between the two supports 34 and the inner surface of the generally annular wall 20.

[0066] Since the profile shape of the base surface 28 of the cavity 25 generally matches the profile shape of the interior surface 38 of the movable wall member 12, the volume within the cavity 25 which can be filled with exhaust gas is significantly reduced with respect to known arrangements. This is now discussed with reference to Figures 2b and 4b, which show, respectively, an enlarged portion of a cross-section of a known turbocharger and a perspective view of an axial end of the bearing housing of the known turbocharger. In Figures 2b and 4b features that are generally equivalent to and substantially the same as features of the turbocharger 1 according to an embodiment of the present invention have the same reference numerals (and will not be described further here). In Figures 2b and 4b features that generally correspond to features of the turbocharger 1 according to an embodiment of the present invention but which differ from those corresponding features have the same reference numerals but with a prime (for example bearing housing 4' generally corresponds to but is different from bearing housing 4).

[0067] As shown in Figures 2b and 4b, in known arrangements the cavity 25' is typically formed as a generally annular channel extending axially into an axially facing surface of the bearing housing 4', comprising: a radially inner curved wall 26, a radially outer curved wall 27 and a generally flat base wall 28'. Therefore, with such a prior art arrangement, when the moveable wall member 12 is disposed in the fully opened position (in which the distal ends of the radially inner and outer flanges 21, 22 may be in contact with the generally flat base wall 28'), apart from the space taken up by the two supports 34 and the two arcuate connecting members 35, the entire volume of the generally annular channel defined by inner surfaces of the radially inner flange 21, the radially outer flange 22 and the generally annular wall 20 can be filled with gas.

[0068] Therefore, the turbocharger 1 that incorporates a variable geometry turbine according to an embodiment of the invention provides of an arrangement with balance apertures 32, 33 that can reduce pressure differences across the movable wall member 12 (i.e. pressure differences between the gas flow through the inlet 11 and the cavity 25 in the housing) whilst reducing the available volume that can support gas within the cavity 25. This is particularly advantageous for situations which, in use, will encounter large fluctuations of pressure within the inlet 11, as now discussed.

[0069] In use, the exhaust gas that flows through the turbine inlet 11 will comprise a plurality of pulses, each pulse originating from a different cylinder of the engine. As a result, the pressure within the turbine inlet 11 fluctuates due to these timing of the exhaust pulses received from the exhaust manifold of the vehicle engine. This pressure fluctuation is present both when the turbocharger is operating in an engine "fired" mode and also an engine "braking" mode.

[0070] The inventors of the present invention have realised that for such a time varying pressure in the turbine inlet 11, although the balance apertures 32, 33 in the movable wall member 12 allow the pressure in the cavity 25 behind the movable wall member 12 to equalise the local pressure in the inlet 11 proximate to the balance apertures 32, 33, there is a time lag between the pressure in the cavity 25 and the local pressure in the inlet 11 proximate to the balance apertures 32, 33. It will be appreciated that the average pressure in the cavity 25 behind the movable wall member 12 will be substantially equal to the local average pressure in the inlet 11 proximate to the balance apertures 32, 33. However, as the instantaneous pressure in the inlet 11 proximate to the balance apertures 32, 33 varies with time (due to the timing of the exhaust pulses), the instantaneous pressure in the cavity 25 behind the movable wall member 12 also varies with time in a similar way but with a lag (or phase difference) with respect to the instantaneous pressure in the inlet 11 proximate to the balance apertures 32, 33. For sufficiently high frequency pressure variations this time lag can result large in time varying loads being applied to the movable wall member 12 (which loads must be overcome by the actuating mechanism in order to accurately control position of the movable wall member 12). Furthermore, the inventors of the present invention have realised that this time lag (which represents the time taken to fill or evacuate the cavity 25 to equalise pressure across the balance apertures 32, 33) is dependent on the volume of the cavity 25 which is filled with gas.

[0071] Since the profile shape of the base surface 28 generally matches the profile shape of the interior surface 38 of the movable wall member 12, the volume within the cavity 25 which can be filled with gas is significantly reduced with respect to known arrangements (as can be seen from a comparison of Figures 2a and 2b). In turn, advantageously, this reduces the magnitude of the peak to peak variation in the loads that are applied to the movable wall member 12 and which must be overcome by the actuating mechanism in order to accurately control position of the movable wall member 12.

[0072] It will be appreciated that although the matching of the profile shape of the base surface 28 to the profile shape of the interior surface 38 of the movable wall member 12 reduces the volume within the cavity 25 which can be filled with gas for a given position of the movable wall member 12, the size of this volume is dependent on the axial position of the movable wall member 12. Figure 5 shows a plot 50 of the volume in the cavity 25 as a function of the axial gap between the generally annular wall 20 and the shroud 13 for the embodiment described above with reference to Figures 1, 2a, 3a, 3b and 4a. Also shown in Figure 5 is a plot 52 of the volume in the cavity 25' as a function of the axial gap between the generally annular wall 20 and the shroud 13 for the known arrangement shown in Figures 2b and 4b. Also shown in Figure 5 is a plot 54 of the volume reduction (as a percentage) of the cavity 25 (relative the known cavity 25') as a function of the axial gap between the generally annular wall 20 and the shroud 13.

[0073] The three plots 50, 52, 54 shown in Figure 5 each show three data points, each one representing a different position of the moveable wall member 12. These three positions are shown in Figures 6a, 6b and 6c. The first position (see Figure 6a) represents approximately zero axial gap between the generally annular wall 20 and the shroud 13 and represents a closed position of the moveable wall member 12. The second position (see Figure 6b) represents a position between the closed and open positions of the moveable wall member 12. The third position (see Figure 6c) represents the maximum axial gap between the generally annular wall 20 and the shroud 13 and represents an open position of the moveable wall member 12. In this particular embodiment, when the moveable wall member 12 is disposed in the fully open position, the axial gap between the generally annular wall 20 and the shroud 13 is approximately 19.6 mm.

[0074] It can be seen that when the moveable wall member 12 is disposed in the fully open position, the provision of the two arcuate radially central portions 40 reduces the available volume behind the movable wall member 12 by approximately 60%. As the moveable wall member 12 moves towards the fully closed position, the reduction in the available volume behind the movable wall member 12 decreases to approximately 30%.

[0075] The variable geometry turbine that forms part of the turbocharger 1 according to an embodiment of the invention reduces the magnitude of the time varying loads that are applied to the movable wall member 12 and which must be overcome by the actuating mechanism without adversely affecting the efficiency of the turbine. These effects can be modelled by applying a pressure trace that may be produced in use by an engine (such a pressure trace may, for example, be measured) as boundary conditions in a simulation of the operation of the turbocharger 1.

[0076] It will be appreciated that the frequency of exhaust pulses through the variable geometry turbine is dependent on the engine speed. The magnitude of the pulses is dependent on the operating mode of the engine (either fired or braking) and the positon of the movable wall member 12. Under braking conditions there is typically a larger pressure drop across the turbine stage (or, equivalently a larger expansion ratio as the exhaust gas moves radially inwards across the face of the generally annular wall member 20). Therefore, generally, a specific set of operating conditions can be characterised by specifying the mode of the engine, the engine speed and the position of the moveable wall member 12.

[0077] Figure 7 shows a plot 56 (dashed line) of the load on the movable wall member 12 as a function of time under fired conditions at an engine speed of 1100 rpm, with an axial gap between the generally annular wall 20 and the shroud 13 of 6.19 mm. As can be seen from Figure 5, an axial gap between the generally annular wall 20 and the shroud 13 of 6.19 mm represents position between the closed and open positions of the moveable wall member 12. Also shown in Figure 7 is a plot 58 (solid line) of the load on the movable wall member 12 as a function of time under the same conditions (fired conditions at an engine speed of 1100 rpm, with an axial gap between the generally annular wall 20 and the shroud 13 of 6.19 mm) but with the known cavity 25' as shown in Figures 2b and 4b.

[0078] It can be seen from Figure 7 that the magnitude 60 of the time varying loads being applied to the movable wall member 12 (which loads must be overcome by the actuating mechanism in order to accurately control position of the movable wall member 12) for the variable geometry turbine shown in Figure 2a is significantly reduced with respect to the magnitude 62 of the time varying loads being applied to the movable wall member 12 for the known variable geometry turbine shown in Figure 2b. For these specific operating conditions, the magnitude 60 of the time varying loads for the variable geometry turbine shown in Figure 2a is reduced by approximately 30% with respect to the magnitude 62 of the time varying loads being applied to the movable wall member 12 for the known variable geometry turbine shown in Figure 2b.

[0079] On the same time scale as for plots 56, 58, Figure 7 also shows a plot 64 (dashed line) of the efficiency of the variable geometry turbine as a function of time under the same conditions (fired conditions at an engine speed of 1100 rpm, with an axial gap between the generally annular wall 20 and the shroud 13 of 6.19 mm). Also shown in Figure 7 is a plot 66 (solid line) of the efficiency of the known variable geometry turbine (as shown in Figures 2b and 4b) as a function of time under the same conditions. It can be seen from the efficiency plots 64, 66 shown in Figure 7 that the arcuate radially central portions 40 do not adversely affect the efficiency of the turbine. In fact, variable geometry turbines according to embodiments of the invention can reduce the magnitude of the time varying loads on the movable wall member 12 and, in addition, can even increase the efficiency of the turbine over known arrangements.

[0080] In some known arrangements, additional "secondary" balance apertures (i.e. similar to the balance apertures 33 shown in Figures 3a and 3b) are provided upstream of (i.e. at a larger radius than) the primary balance apertures, which are disposed between the vanes 15 (i.e. similar to the balance apertures 32 shown in Figures 3a and 3b) so as reduce the time varying load on the moveable wall member 12. In contrast, the variable geometry turbine according to embodiments of the invention do not need such secondary balance apertures 33 and may some embodiments of the invention may have no secondary balance apertures 33. Alternatively, the variable geometry turbine embodiments of the invention may be provided with fewer such secondary balance apertures 33 than known arrangements. It will be appreciated that such secondary balance apertures 33 represent a leak path within the turbine. Therefore, since the variable geometry turbines according to embodiments of the invention do not need such secondary balance apertures 33, or may be provided with fewer such secondary balance apertures 33 than known arrangements, the efficiency of the turbine will be increased relative to such prior art arrangements. In fact, since the profile shape of the base surface 28 generally matches the profile shape of the interior surface 38 of the movable wall member 12, the volume within the cavity 25 which can be filled with gas is significantly reduced with respect to known arrangements. With such a reduced volume within the cavity 25 which can be filled with gas, a smaller total area of the balance apertures 32, 33 can be used with respect to known turbines to achieve the same level of balancing. In turn, this can result in an increase in the efficiency of the turbine with respect to prior art arrangements.

[0081] The load on the movable wall member 12 as a function of time has been investigated under a range of different operating conditions (both fired and braking) and compared with the same but with the known cavity 25' as shown in Figures 2b and 4b.

[0082] Under fired engine conditions, there was no noticeable effect on the predicted mean load on the movable wall member 12. Under braking engine conditions a small shift in the mean load on the movable wall member 12 was observed over the reduced time period considered for analysis (a limited time-period of high frequency exhaust data was run to reduce the time required for the simulation).

[0083] It was found that the peak-to-peak amplitude of the time varying component of the load on the movable wall member 12 was reduced for all cases (both fired and braking) using the modified bearing housing 4. The improvement was more significant for fired mode than braking mode cases. It will be appreciated that under braking conditions the moveable wall member 12 will be positioned such that the axial gap between the generally annular wall 20 and the shroud 13 is relatively small. Furthermore, in such positions, the reduction in the total volume behind the movable wall member 12 is relatively small (see, for example, Figure 5). However, the reduction in the improvement for braking mode cases relative to fired mode cases is greater than one might expect from the change in geometry alone. It is thought that there may be an additional reduction in the effectiveness due to the increased pressure difference (or expansion ratio) experienced across the turbine stage during braking mode operation.

[0084] In order to investigate this, the reduction factor of the amplitude of the time varying component of the load on the movable wall member 12 has been studied as a function of the volume reduction (relative to the known arrangement shown in Figures 2b, 4b). It will be appreciated that the volume reduction can be varied either by varying the position of the movable wall member 12 or by altering its geometry.

[0085] Figure 8 shows the reduction factor of the amplitude of the time varying component of the load on the movable wall member 12 plotted against the volume reduction (relative to the known arrangement shown in Figures 2b, 4b) for 9 different points in the space of operating conditions and geometry of the base wall 28.

[0086] Five of the points 68, 69, 70, 71, 72 correspond to the same geometry as discussed above but for different operating conditions. Three of these points 68, 69, 70 correspond to fired conditions but with different positions of the movable wall member 12. Point 68 corresponds to fired conditions at an engine speed of 1950 rpm, with an axial gap between the generally annular wall 20 and the shroud 13 of 10.93 mm; point 69 corresponds to fired conditions at an engine speed of 1700 rpm, with an axial gap between the generally annular wall 20 and the shroud 13 of 9.58 mm; and point 70 corresponds to fired conditions at an engine speed of 1100 rpm, with an axial gap between the generally annular wall 20 and the shroud 13 of 6.19 mm. Two of these points 71, 72 correspond to braking conditions but with different positions of the movable wall member 12. Point 71 corresponds to braking conditions at an engine speed of 2200 rpm, with an axial gap between the generally annular wall 20 and the shroud 13 of 2.55 mm; and point 72 corresponds to braking conditions at an engine speed of 1800 rpm, with an axial gap between the generally annular wall 20 and the shroud 13 of 0.414 mm

[0087] The remaining points 73, 74, 75, 76 correspond to modified geometries of the base surface 28, with the arcuate radially central portions 40 being either smaller or larger in axial extent than the geometry discussed above.

[0088] Point 73 corresponds to the same operating conditions as point 70 but with the arcuate radially central portions 40 being smaller in axial extent. Similarly, point 74 corresponds to the same operating conditions as point 69 but with the arcuate radially central portions 40 being smaller in axial extent. Point 75 corresponds to the same operating conditions as point 71 but with the arcuate radially central portions 40 being larger in axial extent. Similarly, point 76 corresponds to the same operating conditions as point 71 but with the arcuate radially central portions 40 being smaller in axial extent.

[0089] It can be seen from Figure 8 that the amplitude of the time varying component of the load on the movable wall member 12 was reduced in all cases. The points 68, 69, 70, 73, 74 that correspond to fired conditions follow a first trend that the reduction factor of the amplitude of the time varying component of the load on the movable wall member 12 is proportional to the volume reduction (relative to the known arrangement shown in Figures 2b, 4b). The points 71, 72, 75, 76 that correspond to braking conditions follow a second trend that the reduction factor of the amplitude of the time varying component of the load on the movable wall member 12 is proportional to the volume reduction (relative to the known arrangement shown in Figures 2b, 4b). It can be seen from these two trends that the improvement is more significant for fired mode conditions than for braking mode conditions.

[0090] It will be appreciated that for a profile shape of the base surface 28 of the cavity 25 in the housing of the variable geometry turbine to generally match a profile shape of the interior surface 38 of the movable wall member 12, the profile shape of the base surface 28 of the cavity 25 should be generally complementary to the profile shape of the interior surface 38 of the movable wall member 12. It will be appreciated that two shapes may generally match, or be generally complementary, if one shape is generally concave and the other shape is generally convex and the convex shape can be partially received within the concave shape.

[0091] In the above described embodiments the matching of the profile shape of the base surface 28 of the cavity 25 in the housing to the profile shape of the interior surface 38 of the movable wall member 12, is achieved by providing axial protrusions 40 from the base surface 28 of the cavity 25 that are received in, and generally match, an interior of the movable wall member 12. However, it will be appreciated that in additionally or alternatively, in some embodiments the shape of the interior of the movable wall member 12 may be modified to match the profile of the base surface 28 of the cavity 25.

[0092] It will be appreciated that the bearing housing 4 and the movable wall member 12 are both formed from materials that are impermeable to gas flow. For example, the bearing housing 4 and the movable wall member 12 may both be formed from steel. In particular, arcuate radially central portions 40, which are of the form of axial protrusions from a generally flat portion 39 of the base surface 28 at an axial end surface of the bearing housing 4, are formed from a material that is impermeable to gas flow (for example steel). It will be appreciated that the arcuate radially central portions 40, may be integrally formed with the bearing housing 4. For example, they may be formed therewith during a casting process. Optionally, they may be at least partially formed by a machining process following a casting process. It will be appreciated that the reduction in the amplitude of the time varying loads that are applied to the movable wall member 12 is achieved by reducing the available volume behind the movable wall member 12 in which gas can flow. It is known to provide a filter material in the cavity behind the movable wall member 12 that can capture particulate matter entrained with exhaust gases flowing through the turbine of a variable geometry turbocharger and can facilitate the oxidation of such particulate matter to (gaseous) carbon dioxide and water. However, such filter materials are permeable to fluid flow and may, for example, comprise a mesh of wire. Due to the low density of such wire mesh materials, they typically do not significantly reduce the available volume that can receive exhaust gases and therefore would not enjoy any significant reduction in the amplitude of time varying loads on the movable wall member 12.

[0093] It will be appreciated that it is desirable to reduce the available volume behind the moveable wall member 12 that can support exhaust gases as much as possible. Preferably, the shape of the base surface 28 of the cavity 25 in the housing of the variable geometry turbine and the profile shape of the interior surface 38 of the movable wall member 12 are such that at the volume of the cavity is reduced by at least 20% relative to an arrangement wherein the base surface 28 of the cavity and the interior surface of the generally annular wall 20 are both flat (as in Figure 2b). More preferably, the shape of the base surface 28 of the cavity 25 in the housing of the variable geometry turbine and the profile shape of the interior surface 38 of the movable wall member 12 are such that at the volume of the cavity is reduced by at least 30% relative to an arrangement wherein the base surface 28 of the cavity and the interior surface of the generally annular wall 20 are both flat (as in Figure 2b). More preferably, the shape of the base surface 28 of the cavity 25 in the housing of the variable geometry turbine and the profile shape of the interior surface 38 of the movable wall member 12 are such that at the volume of the cavity is reduced by at least 40% relative to an arrangement wherein the base surface 28 of the cavity and the interior surface of the generally annular wall 20 are both flat (as in Figure 2b). More preferably, the shape of the base surface 28 of the cavity 25 in the housing of the variable geometry turbine and the profile shape of the interior surface 38 of the movable wall member 12 are such that at the volume of the cavity is reduced by at least 50% relative to an arrangement wherein the base surface 28 of the cavity and the interior surface of the generally annular wall 20 are both flat (as in Figure 2b). More preferably, the shape of the base surface 28 of the cavity 25 in the housing of the variable geometry turbine and the profile shape of the interior surface 38 of the movable wall member 12 are such that at the volume of the cavity is reduced by at least 60% relative to an arrangement wherein the base surface 28 of the cavity and the interior surface of the generally annular wall 20 are both flat (as in Figure 2b).

[0094] According to an embodiment of the present invention there is provided a method of forming a variable geometry turbine substantially as described above with reference to the turbocharger 1 of Figures 1, 2a, 3a, 3b and 4a. In particular, embodiments of the present invention may relate to methods of forming the cavity 25 and/or the part of the bearing housing that defines the cavity 25 (i.e. the bearing housing 4). The method may further comprise mounting a movable wall member 12 in the cavity 25 of the housing 4 such that the movable wall member is axially movable relative to the housing.

[0095] In some embodiments, the bearing housing 4 may be cast with the cavity 25 having a base surface 28 as described above. For example, the entire base surface 28, including the the arcuate radially central portions 40 may be formed by such a casting process.

[0096] The method of forming the bearing housing 4 may further comprise machining the casting to form at least a part of the cavity 25. For example, the casting may not define the cavity 25 or, alternatively, may only partially define the cavity 25. Additional machining steps (for example milling) may be used to define, or further define, the cavity 25 having a suitable profile shape.

[0097] Additionally or alternatively, the method of forming the bearing housing 4 may further comprise attaching to the casting one or more additional members, the one or more additional members contributing to the profile shape of the base surface of the cavity. For example, the casting may form a cavity having a base surface which has a profile shape that does not match the profile shape of the interior surface 38 of the movable wall member 12 and one or more additional members may be attached (for example via bolts, screws, rivets or any other suitable fastener) to alter the shape of the base surface of the cavity such that it does substantially match the profile shape of the interior surface 38 of the movable wall member 12. For example, a casting process may be used to form a cavity 25' having a generally flat base surface 28' (i.e. as shown in Figure 2b). Subsequently, additional filler members may be attached to this flat base surface 28'. For example, the additional filler members may be generally of the form of the arcuate radially central portions 40 described above. It will be appreciated that each such arcuate radially central portion 40 may be formed from a plurality of additional members that are attached to a casting.

[0098] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A variable geometry turbine comprising:

a housing (2, 4);

a turbine wheel (6) supported in the housing for rotation about an Z axis (8),

a movable wall member (12) comprising a generally annular wall (20) and radially inner and outer flanges (21, 22) extending axially from the generally annular wall;

a cavity (25) provided in the housing for receipt of the radially inner and outer flanges of the movable wall member, the movable wall member being axially movable relative to the housing to vary the extent to which the radially inner and outer flanges of the movable wall member are received in the cavity, the cavity being defined by radially inner and outer curved side surfaces (26, 27) and a base surface Z (28) extending between the radially inner and outer curved side surfaces;

an inlet passageway (11) extending radially inwards towards the turbine wheel and defined between a face of the generally annular wall of the movable wall member and an opposing wall of the housing, such that said axial movement of the movable wall member relative to the housing varies the axial width of the inlet passageway; and

a plurality of axially extending apertures (32, 33) provided through the generally annular wall of the movable wall member, such that the inlet and the cavity are in fluid communication via the plurality of apertures;

wherein a profile shape of the base surface generally matches a profile shape of an interior surface (38) of the movable wall member; and

wherein the base surface of the cavity comprises at least one arcuate radially central portion (40) which is shaped so as to be received in an interior of the movable wall member when it is disposed in a fully open position.

2. The variable geometry turbine of claim 1 wherein the interior surface of the movable wall member is at least partially defined by inner surfaces of the generally annular wall and the radially inner and outer flanges.

3. The variable geometry turbine of claim 1 or claim 2 wherein the movable wall member further comprises at least one support (34), wherein optionally the interior surface of the movable wall member is at least partially defined by said at least one support and any connecting members (35) or connecting portions of said at least one support.

4. The variable geometry turbine of any preceding claim wherein at least part of the base surface of the cavity and at least part of the interior surface of the movable wall member is not flat.

5. The variable geometry turbine of any preceding claim wherein one of the base surface of the cavity and the interior surface of the movable wall member is at least partially generally concave and the other is at least partially generally convex, and wherein the generally convex shape can be partially received within the generally concave shape.

6. The variable geometry turbine of any preceding claim wherein:

each arcuate radially central portion is of the form of an axial protrusion from a generally flat portion (39) J Z of the base surface; and/or

each arcuate radially central portion extends circumferentially generally between apertures (36) that a support of the movable wall member extends through.

7. The variable geometry turbine of claim 6 wherein along its circumferential extent, each arcuate radially central portion comprises two end portions (43) and a central portion (44) disposed therebetween, the axial extent of the central portion being greater than that of the two end portions.

8. The variable geometry turbine of claim 7 wherein adjacent end portions of two arcuate radially central portions are separated by an aperture through which a support of the movable wall member extends and wherein the reduced axial extent of two end portions relative to the central portion forms a void that accommodates a connecting member or portion of said support.

9. The variable geometry turbine of any preceding claim wherein the movable wall member supports an array of circumferentially spaced inlet vanes each of which extends across the inlet passageway, wherein optionally at least some of the axially extending apertures provided through the generally annular wall of the movable wall member are located between the inlet vanes.

10. The variable geometry turbine of any preceding claim wherein the movable wall member is moveable between a fully open position and a fully closed position and wherein when disposed in the fully open position part of the movable wall member contacts part of the base surface of the cavity.

11. The variable geometry turbine of any preceding claim wherein the base surface of the cavity and the interior surface of the movable wall member are formed from materials that are impermeable to gas flow.

12. The variable geometry turbine of any preceding claim wherein the shape of the base surface of the cavity and the profile shape of the interior surface of the movable wall member are such that the volume of the cavity is reduced by at least 20% relative to an arrangement wherein the base surface of the cavity and the interior surface of the generally annular wall were both flat.

13. A turbocharger (1) comprising the variable geometry turbine of any preceding claim.

14. A method of forming a variable geometry turbine comprising:

providing a movable wall member (12) comprising a generally annular wall (20) and radially inner and outer flanges (21, 22) extending axially from the generally annular wall;

providing a housing (2, 4) J Z having a cavity (25) for receipt of the radially inner and outer flanges of the movable wall member, the cavity being defined by radially inner and outer curved side surfaces (26, 27) and a base surface (28) extending between the radially inner and outer curved side surfaces;

mounting the movable wall member in the cavity of the housing such that the movable wall member being axially movable relative to the housing to vary the extent to which the radially inner and outer flanges of the movable wall member are received in the cavity;

mounting a turbine wheel (6) in the housing for rotation about an Z axis (8), such that a face of the generally annular wall of the movable wall member and an opposing wall of the housing define an inlet passageway (11) extending radially inwards towards the turbine wheel; and

wherein a plurality of axially extending apertures (32, 33) are provided through the generally annular wall of the movable wall member, such that the inlet and the cavity are in fluid communication via the plurality of apertures; and

wherein a profile shape of the base surface generally matches a profile shape of an interior surface of the movable wall member; and

wherein the base surface of the cavity comprises at least one arcuate radially central portion (70) which is shaped so as to be received in an interior of the movable wall member when it is disposed in a fully open position.

15. The method of claim 14 wherein providing the housing having the cavity comprises casting a part of the housing on which the cavity is formed wherein:

optionally, providing the housing having the cavity further comprises machining the casting to form at least a part of the cavity; and/or

optionally, providing the housing having the cavity further comprises attaching to the casting one or more additional members, the one or more additional members contributing to the profile shape of the base surface of the cavity.

Ansprüche

1. Turbine mit variabler Geometrie, die Folgendes umfasst:

ein Gehäuse (2, 4);

ein Turbinenrad (6), das im Gehäuse gestützt wird, zur Drehung um eine Achse (8),

ein bewegbares Wandelement (12), das eine im Allgemeinen ringförmige Wand (20) und einen radial inneren und äußeren Flansch (21, 22), die sich axial von der im Allgemeinen ringförmigen Wand erstrecken, umfasst;

einen Hohlraum (25), der im Gehäuse zur Aufnahme des radial inneren und äußeren Flansches des bewegbaren Wandelements bereitgestellt ist, wobei das bewegbare Wandelement relativ zum Gehäuse axial bewegbar ist, um den Umfang, in dem der radial innere und äußere Flansch des bewegbaren Wandelements im Hohlraum aufgenommen werden, zu variieren, wobei der Hohlraum durch eine radial innere und äußere gekrümmte Seitenfläche (26, 27) und eine Basisfläche (28), die sich zwischen der radial inneren und äußeren gekrümmten Seitenfläche erstreckt, definiert ist;

einen Einlasskanal (11), der sich radial nach innen zum Turbinenrad erstreckt und zwischen einer Seite der im Allgemeinen ringförmigen Wand des bewegbaren Wandelements und einer gegenüberliegenden Wand des Gehäuses definiert ist, derart, dass die Axialbewegung des bewegbaren Wandelements relativ zum Gehäuse die axiale Breite des Einlasskanals variiert; und

eine Vielzahl von sich axial erstreckenden Öffnungen (32, 33), die durch die im Allgemeinen ringförmige Wand des bewegbaren Wandelements bereitgestellt sind, derart, dass der Einlass und der Hohlraum durch die Vielzahl von Öffnungen in Fluidkommunikation stehen;

wobei eine Profilform der Basisfläche im Allgemeinen mit einer Profilform einer Innenfläche (38) des bewegbaren Wandelements übereinstimmt und

wobei die Basisfläche des Hohlraums mindestens einen bogenförmigen radial zentralen Abschnitt (40) umfasst, der derart geformt ist, dass er in einem Innern des bewegbaren Wandelements aufgenommen ist, wenn sie in einer vollständig offenen Position angeordnet ist.

2. Turbine mit variabler Geometrie nach Anspruch 1, wobei die Innenfläche des bewegbaren Wandelements mindestens teilweise durch innere Flächen der im Allgemeinen ringförmigen Wand und den radial inneren und äußeren Flansch definiert wird.

3. Turbine mit variabler Geometrie nach Anspruch 1 oder Anspruch 2, wobei das bewegbare Wandelement ferner mindestens eine Stütze (34) umfasst, wobei wahlweise die Innenfläche des bewegbaren Wandelements mindestens teilweise durch die mindestens eine Stütze und beliebige Verbindungselemente (35) oder Verbindungsabschnitte der mindestens einen Stütze definiert wird.

4. Turbine mit variabler Geometrie nach einem der vorhergehenden Ansprüche, wobei mindestens ein Teil der Basisfläche des Hohlraums und mindestens ein Teil der Innenfläche des bewegbaren Wandelements nicht eben sind.

5. Turbine mit variabler Geometrie nach einem der vorhergehenden Ansprüche, wobei eine der Basisfläche des Hohlraums und der Innenfläche des bewegbaren Wandelements mindestens teilweise im Allgemeinen konkav und die andere mindestens teilweise im Allgemeinen konvex ist und wobei die im Allgemeinen konvexe Form teilweise in die im Allgemeinen konkave Form aufgenommen werden kann.

6. Turbine mit variabler Geometrie nach einem der vorhergehenden Ansprüche, wobei:

jeder bogenförmige radial zentrale Abschnitt die Ausformung eines axialen Vorsprungs von einem im Allgemeinen ebenen Abschnitt (39) der Basisfläche aufweist und/oder

jeder bogenförmige radial zentrale Abschnitt sich umfänglich erstreckt, im Allgemeinen zwischen Öffnungen (36), durch die sich eine Stütze des bewegbaren Wandelements erstreckt.

7. Turbine mit variabler Geometrie nach Anspruch 6, wobei jeder bogenförmige radial zentrale Abschnitt entlang seiner umfänglichen Erstreckung zwei Endabschnitte (43) und einen zentralen Abschnitt (44), der dazwischen angeordnet ist, umfasst, wobei die axiale Erstreckung des zentralen Abschnitts größer ist als die der zwei Endabschnitte.

8. Turbine mit variabler Geometrie nach Anspruch 7, wobei benachbarte Endabschnitte von zwei bogenförmigen radial zentralen Abschnitten durch eine Öffnung getrennt sind, durch die sich eine Stütze des bewegbaren Wandelements erstreckt, und wobei die reduzierte axiale Erstreckung von zwei Endabschnitten relativ zum zentralen Abschnitt eine Leere bildet, in der ein Verbindungselement oder -abschnitt der Stütze aufgenommen ist.

9. Turbine mit variabler Geometrie nach einem der vorhergehenden Ansprüche, wobei das bewegbare Wandelement eine Gruppe von umfänglich beabstandeten Einlassschaufeln stützt, von denen sich jede über den Einlasskanal erstreckt, wobei sich wahlweise mindestens einige der sich axial erstreckenden Öffnungen, die durch die im Allgemeinen ringförmige Wand des bewegbaren Wandelements bereitgestellt sind, zwischen den Einlassschaufeln befinden.

10. Turbine mit variabler Geometrie nach einem der vorhergehenden Ansprüche, wobei das bewegbare Wandelement zwischen einer vollständig offenen Position und einer vollständig geschlossenen Position bewegbar ist und wobei bei Anordnung in der vollständig offenen Position ein Teil des bewegbaren Wandelements einen Teil der Basisfläche des Hohlraums berührt.

11. Turbine mit variabler Geometrie nach einem der vorhergehenden Ansprüche, wobei die Basisfläche des Hohlraums und die Innenfläche des bewegbaren Wandelements aus Materialien gebildet sind, die für einen Gasstrom undurchdringbar sind.

12. Turbine mit variabler Geometrie nach einem der vorhergehenden Ansprüche, wobei die Form der Basisfläche des Hohlraums und die Profilform der Innenfläche des bewegbaren Wandelements derart sind, dass das Volumen des Hohlraums relativ zu einer Anordnung, in der die Basisfläche des Hohlraums und die Innenfläche der im Allgemeinen ringförmigen Wand beide eben wären, um mindestens 20 % reduziert ist.

13. Turbolader (1), der die Turbine mit variabler Geometrie nach einem der vorhergehenden Ansprüche umfasst.

14. Verfahren zum Bilden einer Turbine mit variabler Geometrie, das Folgendes umfasst:

Bereitstellen eines bewegbaren Wandelements (12), das eine im Allgemeinen ringförmige Wand (20) und einen radial inneren und äußeren Flansch (21, 22), die sich axial von der im Allgemeinen ringförmigen Wand erstrecken, umfasst;

Bereitstellen eines Gehäuses (2, 4) mit einem Hohlraum (25) zum Aufnehmen des radial inneren und äußeren Flansches des bewegbaren Wandelements, wobei der Hohlraum durch eine radial innere und äußere gekrümmte Seitenfläche (26, 27) und eine Basisfläche (28), die sich zwischen der radial inneren und äußeren gekrümmten Seitenfläche erstreckt, definiert ist;

Montieren des bewegbaren Wandelements im Hohlraum des Gehäuses, derart, dass das bewegbare Wandelement relativ zum Gehäuse axial bewegbar ist, um den Umfang, in dem der radial innere und äußere Flansch des bewegbaren Wandelements im Hohlraum aufgenommen werden, zu variieren;

Montieren eines Turbinenrads (6) im Gehäuse zur Drehung um eine Achse (8), derart, dass eine Seite der im Allgemeinen ringförmigen Wand des bewegbaren Wandelements und eine gegenüberliegende Wand des Gehäuses einen Einlasskanal (11) definieren, der sich radial nach innen zum Turbinenrad erstreckt; und

wobei eine Vielzahl von sich axial erstreckenden Öffnungen (32, 33) durch die im Allgemeinen ringförmige Wand des bewegbaren Wandelements bereitgestellt sind, derart, dass der Einlass und der Hohlraum durch die Vielzahl von Öffnungen in Fluidkommunikation stehen; und

wobei eine Profilform der Basisfläche im Allgemeinen mit einer Profilform einer Innenfläche des bewegbaren Wandelements übereinstimmt und

wobei die Basisfläche des Hohlraums mindestens einen bogenförmigen radial zentralen Abschnitt (70) umfasst, der derart geformt ist, dass er in einem Innern des bewegbaren Wandelements aufgenommen ist, wenn sie in einer vollständig offenen Position angeordnet ist.

15. Verfahren nach Anspruch 14, wobei das Bereitstellen des Gehäuses mit dem Hohlraum das Gießen eines Teils des Gehäuses, an dem der Hohlraum gebildet ist, umfasst, wobei:

wahlweise das Bereitstellen des Gehäuses mit dem Hohlraum ferner das Bearbeiten des Gussteils zum Bilden von mindestens einem Teil des Hohlraums umfasst und/oder

wahlweise das Bereitstellen des Gehäuses mit dem Hohlraum ferner das Befestigen von einem oder mehreren zusätzlichen Elementen am Gussteil umfasst, wobei das eine oder die mehreren zusätzlichen Elemente zur Profilform der Basisfläche des Hohlraums beitragen.

Revendications

1. Turbine à géométrie variable, comprenant :

un carter (2, 4) ;

une roue de turbine (6) supportée dans le carter en vue d'une rotation autour d'un axe (8) ;

un élément de paroi mobile (12) comprenant une paroi généralement annulaire (20) et des brides radialement interne et externe (21, 22) s'étendant axialement à partir de la paroi généralement annulaire ;

une cavité (25) fournie dans le carter pour recevoir les brides radialement interne et externe de l'élément de paroi mobile, l'élément de paroi mobile étant axialement mobile par rapport au carter pour faire varier l'ampleur de la réception des brides radialement interne et externe de l'élément de paroi mobile dans la cavité, la cavité étant définie par des surfaces latérales courbées radialement interne et externe (26, 27) et une surface de base (28) s'étendant entre les surfaces latérales courbées radialement interne et externe ;

un passage d'entrée (11) s'étendant radialement vers l'intérieur, vers la roue de turbine, et défini entre une face de la paroi généralement annulaire de l'élément de paroi mobile et une paroi opposée du carter, de sorte que ledit mouvement axial de l'élément de paroi mobile par rapport au carter modifie la largeur axiale du passage d'entrée ; et

une pluralité d'ouvertures à extension axiale (32, 33) fournies à travers la paroi généralement annulaire de l'élément de paroi mobile, de sorte que l'entrée et la cavité sont en communication fluidique par l'intermédiaire de la pluralité d'ouvertures ;

dans laquelle une forme de profil de la surface de base correspond en général à une forme de profil d'une surface interne (38) de l'élément de paroi mobile ; et

dans laquelle la surface de base de la cavité comprend au moins une partie arquée radialement centrale (40) formée de sorte à être reçue dans un intérieur de l'élément de paroi mobile lors de sa disposition dans une position complètement ouverte.

2. Turbine à géométrie variable selon la revendication 1, dans laquelle la surface interne de l'élément de paroi mobile est au moins en partie définie par des surfaces internes de la paroi généralement annulaire et les brides radialement interne et externe.

3. Turbine à géométrie variable selon la revendication 1 ou la revendication 2, dans laquelle l'élément de paroi mobile comprend en outre au moins un support (34), la surface interne de l'élément de paroi mobile étant optionnellement au moins en partie définie par ledit au moins un support et par de quelconques éléments de raccordement (35) ou parties de raccordement dudit au moins un support.

4. Turbine à géométrie variable selon l'une quelconque des revendications précédentes, dans laquelle au moins une partie de la surface de base de la cavité et au moins une partie de la surface interne de l'élément de paroi mobile ne sont pas plates.

5. Turbine à géométrie variable selon l'une quelconque des revendications précédentes, dans laquelle l'une de la surface de base de la cavité et la surface interne de l'élément de paroi mobile, est au moins en partie généralement concave, l'autre surface étant au moins en partie généralement convexe, et dans laquelle la forme généralement convexe peut être en partie reçue dans la forme généralement concave.

6. Turbine à géométrie variable selon l'une quelconque des revendications précédentes, dans laquelle :

chaque partie arquée radialement centrale a la forme d'une saillie axiale, débordant d'une partie généralement plate (39) de la surface de base ; et/ou

chaque partie arquée radialement centrale s'étend de manière circonférentielle, généralement entre des ouvertures (36) à travers lesquelles s'étend un support de l'élément de paroi mobile.

7. Turbine à géométrie variable selon la revendication 6, dans laquelle, le long de son extension circonférentielle, chaque partie arquée radialement centrale comprend deux parties d'extrémité (43) et une partie centrale (44) disposée entre elles, l'extension axiale de la partie centrale étant supérieure à celle des deux parties d'extrémité.

8. Turbine à géométrie variable selon la revendication 7, dans laquelle des parties d'extrémité adjacentes de deux parties arquées radialement centrales sont séparées par une ouverture à travers laquelle s'étend un support de l'élément de paroi mobile, et dans laquelle l'extension axiale réduite de deux parties d'extrémité par rapport à la partie centrale forme un vide recevant un élément ou une partie de raccordement dudit support.

9. Turbine à géométrie variable selon l'une quelconque des revendications précédentes, dans laquelle l'élément de paroi mobile supporte un réseau d'aubes d'entrée à espacement circonférentiel, dont chacune s'étend à travers le passage d'entrée, dans laquelle au moins certaines des ouvertures à extension axiale fournies à travers la paroi généralement annulaire de l'élément de paroi mobile sont agencées entre les aubes d'entrée.

10. Turbine à géométrie variable selon l'une quelconque des revendications précédentes, dans laquelle l'élément de paroi mobile est mobile entre une position complètement ouverte et une position complètement fermée et dans laquelle, lors de la disposition dans la position complètement ouverte, une partie de l'élément de paroi mobile contacte une partie de la surface de base de la cavité.

11. Turbine à géométrie variable selon l'une quelconque des revendications précédentes, dans laquelle la surface de base de la cavité et la surface interne de l'élément de paroi mobile sont formées à partir de matériaux imperméables à l'écoulement de gaz.

12. Turbine à géométrie variable selon l'une quelconque des revendications précédentes, dans laquelle la forme de la surface de base de la cavité et la forme de profil de la surface interne de l'élément de paroi mobile sont telles que le volume de la cavité est réduit d'au moins 20% par rapport à un agencement dans lequel la surface de base de la cavité et la surface interne de la paroi généralement annulaire étaient toutes les deux plates.

13. Turbocompresseur (1) comprenant la turbine à géométrie variable selon l'une quelconque des revendications précédentes.

14. Procédé de fabrication d'une turbine à géométrie variable, comprenant les étapes ci-dessous :

fourniture d'un élément de paroi mobile (12) comprenant une paroi généralement annulaire (20) et des brides radialement interne et externe (21, 22) s'étendant axialement à partir de la paroi généralement annulaire ;

fourniture d'un carter (2, 4) comportant une cavité (25) pour recevoir les brides radialement interne et externe de l'élément de paroi mobile, la cavité étant définie par des surfaces latérales courbées radialement interne et externe (26, 27) et une surface de base (28) s'étendant entre les surfaces latérales courbées radialement interne et externe ;

montage de l'élément de paroi mobile dans la cavité du carter de manière à ce que l'élément de paroi mobile est mobile axialement par rapport au carter pour faire varier l'ampleur de la réception des brides radialement interne et externe de l'élément de paroi mobile dans la cavité ;

montage d'une roue de turbine (6) dans le carter en vue d'une rotation autour d'un axe (8), de sorte qu'une face de la paroi généralement annulaire de l'élément de paroi mobile et une paroi opposée du carter définissent un passage d'entrée (11) s'étendant radialement vers l'intérieur, vers la roue de turbine ; et

dans lequel une pluralité d'ouvertures à extension axiale (32, 33) sont fournies à travers la paroi généralement annulaire de l'élément de paroi mobile, de sorte que l'entrée et la cavité sont en communication fluidique par l'intermédiaire de la pluralité d'ouvertures ; et

dans lequel une forme de profil de la surface de base correspond en général à une forme de profil d'une surface interne de l'élément de paroi mobile ; et

dans lequel la surface de base de la cavité comprend au moins une partie arquée généralement centrale (70) formée de sorte à être reçue dans un intérieur de l'élément de paroi mobile lors de sa disposition dans une position complètement ouverte.

15. Procédé selon la revendication 14, dans lequel l'étape de fourniture du carter comportant la cavité comprend le moulage d'une partie du carter sur laquelle la cavité est formée ; dans lequel :

optionnellement, l'étape de fourniture du carter comportant la cavité comprend en outre l'usinage du moulage pour former au moins une partie de la cavité ; et/ou

optionnellement, l'étape de fourniture du carter comportant la cavité comprend la fixation sur le moulage d'un ou de plusieurs éléments additionnels, l'un ou les plusieurs éléments additionnels contribuant à la forme de profil de la surface de base de la cavité.

Drawing